The Interstellar medium is made mostly of large gas clouds
which can be of the order of 50 parsecs (3.086 4
1016m)across. Gas in the interstellar medium is more difficult to detect than
dust as it does not cause the general extinction of light. The existence of
interstellar gas was first predicted by Johannes Hartmann in 1904, when he
observed that certain absorption lines of binary stars were not Doppler shifted,
he concluded that this was due giant gas clouds.

Some of the effects of interstellar gas clouds can be seen on
these pages: